1. Field of the Invention
The present invention relates to a switching amplifier that performs pulse amplification by using as a switching control signal a quantized signal obtained by delta-sigma modulation.
2. Description of Related Art
An example of the configuration of a conventional switching amplifier is shown in FIG. 10. The conventional switching amplifier shown in FIG. 10 is a switching amplifier that performs pulse amplification by using as a switching control signal a quantized signal obtained by delta-sigma modulation, and is composed of a delta-sigma modulation circuit 1, a pulse amplifier 6, a low-pass filter 7, and an attenuator 8. The delta-sigma modulation circuit 1 consists of an adder 2, a group of integrators 3, an adder 4, and a quantizer 5.
The delta-sigma modulation circuit 1 converts an input signal SIN that is either an analog signal or a multi-bit signal into a 1-bit signal, and then sends the 1-bit signal to the pulse amplifier 6. The pulse amplifier 6 has a switching element (not shown) such as a FET. The pulse amplifier 6 performs power amplification of the 1-bit signal by switching the switching element according to the 1-bit signal, and then sends the 1-bit signal thus power-amplified to the low-pass filter 7 and the attenuator 8. The low-pass filter 7 eliminates high-frequency components from the output signal of the pulse amplifier 6, and then outputs an output signal SOUT that is an analog signal. The output signal SOUT thus obtained serves as an output signal of the switching amplifier. On the other hand, the output signal of the pulse amplifier 6 is attenuated by the attenuator 8, and is then negatively fed back to the delta-sigma modulation circuit 1.
When the delta-sigma modulation circuit is made to operate at a higher sampling frequency, it is possible to make the oscillation threshold higher, and improve the S/N ratio. However, the problem here is that, in the conventional switching amplifier shown in FIG. 10, when the delta-sigma modulation circuit 1 is made to operate at a higher sampling frequency, the number of switching operations performed by the switching element in the pulse amplifier 6 is increased. This increases switching losses, leading to lower electric power efficiency. Therefore, from the viewpoint of energy saving, it is undesirable to increase unnecessarily the sampling frequency in the delta-sigma modulation circuit 1 because it results in higher electric power consumption in the switching amplifier.
Japanese Patent Application Laid-Open No. 2004-80152 discloses a delta-sigma modulator that can use a sampling frequency while switching it from one frequency to another, but does not disclose which criterion is used to switch a sampling frequency. On the other hand, Japanese Patent Application Laid-Open No. 2003-60508 discloses a delta-sigma modulator that can use a sampling frequency while switching it from one frequency to another according to a type of a signal (audio or voice) to be processed, but does not give any consideration to the oscillation threshold.